为什么要使用双重间接?或者为什么使用指向指针的指针? [英] Why use double indirection? or Why use pointers to pointers?

问题描述

什么时候应该在 C 中使用双重间接寻址?谁能举个例子解释一下?

When should a double indirection be used in C? Can anyone explain with a example?

我所知道的是，双重间接引用是指向指针的指针.为什么我需要一个指向指针的指针?

What I know is that a double indirection is a pointer to a pointer. Why would I need a pointer to a pointer?

推荐答案

如果你想要一个字符列表(一个单词)，可以使用 char *word

If you want to have a list of characters (a word), you can use char *word

如果你想要一个单词列表(一个句子)，你可以使用char **sentence

If you want a list of words (a sentence), you can use char **sentence

如果你想要一个句子列表(一个独白)，你可以使用char ***monologue

If you want a list of sentences (a monologue), you can use char ***monologue

如果你想要一个独白的列表(传记)，你可以使用char ****biography

If you want a list of monologues (a biography), you can use char ****biography

如果你想要一个传记列表(一个bio-library)，你可以使用char *****biolibrary

If you want a list of biographies (a bio-library), you can use char *****biolibrary

如果你想要一个生物库列表(a ??lol)，你可以使用char ******lol

If you want a list of bio-libraries (a ??lol), you can use char ******lol

... ...

^{是的，我知道这些可能不是最好的数据结构}

一个非常非常无聊的用法示例lol

Usage example with a very very very boring lol

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int wordsinsentence(char **x) {
    int w = 0;
    while (*x) {
        w += 1;
        x++;
    }
    return w;
}

int wordsinmono(char ***x) {
    int w = 0;
    while (*x) {
        w += wordsinsentence(*x);
        x++;
    }
    return w;
}

int wordsinbio(char ****x) {
    int w = 0;
    while (*x) {
        w += wordsinmono(*x);
        x++;
    }
    return w;
}

int wordsinlib(char *****x) {
    int w = 0;
    while (*x) {
        w += wordsinbio(*x);
        x++;
    }
    return w;
}

int wordsinlol(char ******x) {
    int w = 0;
    while (*x) {
        w += wordsinlib(*x);
        x++;
    }
    return w;
}

int main(void) {
    char *word;
    char **sentence;
    char ***monologue;
    char ****biography;
    char *****biolibrary;
    char ******lol;

    //fill data structure
    word = malloc(4 * sizeof *word); // assume it worked
    strcpy(word, "foo");

    sentence = malloc(4 * sizeof *sentence); // assume it worked
    sentence[0] = word;
    sentence[1] = word;
    sentence[2] = word;
    sentence[3] = NULL;

    monologue = malloc(4 * sizeof *monologue); // assume it worked
    monologue[0] = sentence;
    monologue[1] = sentence;
    monologue[2] = sentence;
    monologue[3] = NULL;

    biography = malloc(4 * sizeof *biography); // assume it worked
    biography[0] = monologue;
    biography[1] = monologue;
    biography[2] = monologue;
    biography[3] = NULL;

    biolibrary = malloc(4 * sizeof *biolibrary); // assume it worked
    biolibrary[0] = biography;
    biolibrary[1] = biography;
    biolibrary[2] = biography;
    biolibrary[3] = NULL;

    lol = malloc(4 * sizeof *lol); // assume it worked
    lol[0] = biolibrary;
    lol[1] = biolibrary;
    lol[2] = biolibrary;
    lol[3] = NULL;

    printf("total words in my lol: %d
", wordsinlol(lol));

    free(lol);
    free(biolibrary);
    free(biography);
    free(monologue);
    free(sentence);
    free(word);
}

输出:

total words in my lol: 243

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